Nuclear Magnetic Resonance (NMR) systems have been in use for many years and can be used to provide imaging and/or analysis of a sample being tested. For example, U.S. Pat. No. 6,160,398, U.S. Pat. No. 7,466,128, U.S. Pat. No. 7,986,143, U.S. patent application Ser. No. 12/914,138, and U.S. patent application Ser. No. 13/104,721 describe a variety of NMR technologies, and are incorporated herein by reference. Various different types of NMR include medical NMR, often referred to as Magnetic Resonance Imaging (MRI), and NMR for measuring properties of earth formations, which provides, for example, geophysical techniques for detecting properties of the earth's crust. This disclosure relates to the latter type of NMR, and so the term “NMR” as used herein refers to NMR in the geophysical context. While there is some overlap in the technologies that may be applied in MRI and NMR, the samples being measured and the environments in which measurements are performed are different, leading to many differences in the technologies applied.
In general, NMR measurement involves generating a static magnetic field within a sample volume, emitting Radio-Frequency (RF) electromagnetic pulses into the sample volume, and detecting RF NMR responses from the sample volume. Most commonly, NMR measurement involves emitting multiple RF pulses in rapid succession and measuring the RF NMR responses between the RF pulses. The measured RF NMR responses provide useful information about the sample volume.
NMR measurements may be used to detect properties including, for example, the abundance of hydrogen contained within a sample volume as well as fluid composition, porosity, and permeability of the sample volume. NMR measurements may also be used to detect certain other atomic species, including carbon and potassium.
The term NMR as used herein includes Nuclear Quadrupole Resonance (NQR), unless stated otherwise. NQR is often useful for detecting nitrogen, chlorine and other compounds. NQR measurement techniques are similar to those used for NMR in general. While NMR measurements generally include generating a static magnetic field, a static magnetic field is not always required or used for NQR measurements.
NMR logging is an established type of surface NMR measurement wherein an NMR measurement apparatus is lowered into a borehole in the earth, and NMR measurements are performed to determine properties within and/or surrounding the borehole. However, existing NMR logging apparatus have a number of drawbacks, including high expense and difficulty of operation.
For example, some previous NMR logging apparatus may include most or all apparatus components, including for example a controller, a power supply, a power amplifier, transmit and receive antennae, and Analog to Digital (A/D) converter(s), within a downhole sensor package designed to fit within a borehole. Such a downhole sensor package may be 5 inches or larger in diameter and may weigh from several hundred pounds to over one thousand pounds.
In addition to size and weight limitations, previous NMR logging apparatus do not enable economical long-term or wide area in-situ monitoring of subsurface properties. Long-term or wide area in-situ monitoring is uneconomical when many expensive components are included within a downhole sensor package, because either many expensive NMR logging apparatus must be purchased, or expensive components must be either left in place long-term, and therefore unavailable for other uses, or NMR logging apparatus must be repeatedly mobilized which involves mobilization and human labor costs.